1. Field of the Invention
The present invention relates to a phototransistor used for an image sensor, and more particularly, to a phototransistor capable of reducing a dark current that occurs in the phototransistor and improving sensitivity at low luminance without crosstalk with a neighboring pixel or an image lag by including a buried collector.
2. Description of the Related Art
An image sensor is used to measure strength of light. An image sensor with high sensitivity is constructed with a plurality of phototransistors. In general, the image sensor with high sensitivity uses phototransistors having a laminated or integrated structure.
A phototransistor generates an output current proportional to strength of incident light. Photons which are absorbed in a junction region between a collector and a base or a junction region between an emitter and a base of the phototransistor generate electron-hole pairs collected by a PN junction. Minority carriers collected by the junction operate as a base current. The base current is multiplexed based on a transistor gain, thereby generating a collector current. In addition, an emitter current is a sum of the base current and the collector current and generally used as an output current.
FIG. 1 illustrates a phototransistor having a laminated structure according to a conventional technique.
As shown in FIG. 1, a conventional phototransistor having a laminated structure is constructed with an n+ emitter 115, a p type base 113, and an n type collector 112. In addition, the conventional phototransistor may further include an intrinsic absorption layer 114 between the emitter 115 and the base 113. This structure may be constructed with a p+ emitter, an n type base, and a p type collector.
In the aforementioned structure, when a voltage is applied to the emitter 115 and the collector 112, a weak current flows between the base 113 and the collector 112. However, a barrier is formed between the emitter 115 and the base 113. The current is limited due to the barrier. At this time, when photons are incident onto the base 113, electron-hole pairs (EHPs) are generated due to the photons. Electrons move to the collector 112. Holes fill the barrier formed in the base 113.
Accordingly, as the number of holes increases, the potential of the barrier is lowered due to the holes collected in the barrier, thereby generating a current flow from the emitter to the collector in proportion to the decrease in potential. Thus, a current flows. The current introduced into the collector is greater than an optical current due to the electron-hole pairs generated by the incident light, at all times. The aforementioned procedure may be understood by using FIG. 2.
FIG. 2 illustrates operation mechanism of a general phototransistor.
As shown in FIG. 2, when a voltage is applied between an emitter and a collector, a barrier is formed in a region between the emitter and a base (a of FIG. 2). At this time, when light is incident onto the base, electrons move to the collector, and holes are collected in the barrier.
The potential of the barrier is lowered due to the holes collected in the barrier (b of FIG. 2). Accordingly, electrons of the emitter can easily move to the collector. That is, although the number of the collected holes is small, a large current flows.
Since the conventional phototransistor having the laminated structure is manufactured by using a deposition method or a crystal growth method, it is very limited to apply a procedure of manufacturing the conventional phototransistor to a general procedure of manufacturing a complementary metal oxide semiconductor (CMOS).
In addition, since it is difficult to deal with a dark current generated at a surface, a change between pixels is large. Accordingly, it is difficult to apply the conventional phototransistor to an element that requires a uniform pixel characteristic such as an image sensor.
FIG. 3 illustrates a phototransistor having an integrated structure according to a conventional technique.
Referring to FIG. 3, the conventional phototransistor having the integrated structure is constructed with two electrodes poly-1 and poly-2, an n+ emitter, a p type base, and an n type well or collector. In addition, a field oxide layer (FOX) for separating pixels from one another is formed.
At this time, in a state where a negative voltage is applied to the emitter and a negative voltage is applied to an electrode poly-2, when light is incident onto the p type base, electron-hole pairs are generated. Electrons among the generated electron-hole pairs move to the n type well (collector), and holes remain in the base, thereby increasing potential of the base.
When a negative voltage is applied to the electrode poly-2, holes remaining in the base are dragged into the electrode poly-2. Accordingly, holes proportional to an amount of light are collected in the electrode poly-2. In this state, since only a current caused by movement of electrons exists, a weak current flows. This state is referred to as a hole accumulation state.
A procedure of reading an image is achieved by applying a positive or zero voltage to the electrode poly-2. That is, when the positive voltage is applied to the electrode poly-2, a junction between the base and the emitter is forwardly biased. Accordingly, holes collected in the hole accumulation state are pushed out due to repulsive force. At this time, most holes exist between the emitter and the collector. Accordingly, since the barrier between the base and the emitter is lowered due to the holes, a current related to the amount of holes collected in the base flows between the emitter and the collector.
In a case where strong light is incident onto the base of the phototransistor having the aforementioned structure, the base potential is rapidly increased due to the holes generated by the light until the junction between the base and the emitter is weakly biased in the forward direction. In addition, holes that are generated in the base fill a region under the electrode poly-2 and flow to the emitter. Accordingly, a large current flows in an electrode poly-1 in the hole accumulation state. This large current is referred to as an overflow. This large current operates as noise of a neighboring pixel. In addition, since it is difficult to remove holes of a pixel exposed to strong light, an image lag in which bright light is dragged occurs, thereby deteriorating image quality.
On the other hand, the emitter is connected to the base through the surface, and the base is connected to a ground through the surface. Accordingly, a dark current may occur in low luminance.